Oil produced from a significant number of oil reserves around the world is simply too heavy to flow under ambient conditions. This makes it challenging to bring remote, heavy oil resources closer to markets. In order to render such heavy oils flowable, one of the most common methods known in the art is to reduce the viscosity and density by mixing the heavy oil with a sufficient diluent, e.g. naphtha or any other stream with a much lower density than the heavy oil. The diluted crude oil is sent from the production wellhead via pipeline to an upgrading facility where the diluent stream is recovered and recycled back to the production wellhead in a separate pipeline, and the heavy oil is upgraded with suitable technology known in the art (coking, hydrocracking, hydrotreating, etc.) to produce higher-value products for market. Some typical characteristics of these higher-value products include: lower sulfur content, lower metals content, lower total acid number, lower residuum content, higher API gravity, and lower viscosity. Most of these desirable characteristics are achieved by reacting the heavy oil with hydrogen gas at high temperatures and pressures in the presence of a catalyst.
It is known that this diluent addition/removal process has a number of disadvantages. The infrastructure required for the handling and recovery of diluent can be expensive, especially over long distances. Hydrogen-addition processes such as hydrotreating or hydrocracking require significant investments in capital and infrastructure. Hydrogen-addition processes also have high operating costs, since hydrogen production costs are highly sensitive to natural gas prices. Some remote heavy oil reserves may not even have access to sufficient quantities of low-cost natural gas to support a hydrogen plant. These hydrogen-addition processes also generally require expensive catalysts and resource intensive catalyst handling techniques, including catalyst regeneration. In some cases, the refineries and/or upgrading facilities that are located closest to the production site may have neither the capacity nor the facilities to accept the heavy oil. Coking is often practiced at refineries or upgrading facilities. Significant amounts of by-product solid coke are rejected during the coking process, leading to lower liquid hydrocarbon yield. In addition, the liquid products from a coking plant often require further hydrotreating. Furthermore, the volume of the liquid product from the coking process is significantly less than the volume of the feed crude oil.
Processes have been proposed which have overcome these disadvantages by using supercritical water to upgrade a heavy hydrocarbon feedstock into an upgraded hydrocarbon product or synthetic crude with highly desirable properties (low sulfur content, low metals content, lower density (higher API), lower viscosity, lower residuum content, etc.). Such processes require neither external supply of hydrogen nor catalysts, nor do they produce an appreciable coke by-product. In comparison with the more traditional processes for synthetic crude production, advantages of using supercritical water include a high liquid hydrocarbon yield, no need for externally-supplied hydrogen or catalyst, significant increases in API gravity in the upgraded hydrocarbon product, significant viscosity reduction in the upgraded hydrocarbon product, and significant reduction in sulfur, metals, nitrogen, TAN, and MCR (micro-carbon residue) in the upgraded hydrocarbon product.
Despite the advances made using supercritical water to upgrade heavy hydrocarbons, difficulties remain in such a process. For instance, there remains a need to achieve sufficient dispersion of high viscosity hydrocarbons into supercritical water in order to achieve a commercially acceptable productivity level for producing synthetic crude. Under practical operating ranges of temperature and pressure, heavy oil is not totally dissolved in supercritical water. As a result, process development and reactor design must accommodate a two-phase system. It is known that supercritical water inhibits the undesired side reactions that will lead to the formation of dreg or coke by-products, and this is facilitated by good contact between water and oil. Therefore it would be desirable to further improve and optimize the process performance through enhancing water-oil mixing.